1. Field of the Invention
The present invention relates generally to an apparatus and method for deploying a medical device from a minimally invasive delivery system, such as a delivery catheter, and deploying the device within a patient.
2. Description of the Related Art
Percutaneous aortic valve replacement (PAVR) technology is emerging that provides an extremely effective and safe alternative to therapies for aortic stenosis specifically, and aortic disease generally. Historically, aortic valve replacement necessitated open heart surgery with its attendant risks and costs. The replacement of a deficient cardiac valve performed surgically requires first placing the patient under full anesthesia, opening the thorax, placing the patient under extracorporeal circulation or peripheral aorto-venous heart assistance, temporarily stopping the heart, exposing and excising the deficient valve, and then implanting a prosthetic valve in its place. This procedure has the disadvantage of requiring prolonged patient hospitalization, as well as extensive and often painful recovery. Although safe and effective, surgical aortic valve replacement (SAVR) presents advanced complexities and significant costs. For some patients, however, surgery is not an option for one or many possible reasons. As such, a large percentage of patients suffering from aortic disease go untreated.
To address the risks associated with open-heart implantation, devices and methods for replacing a cardiac valve by less invasive means have been developed. For example, CoreValve, Inc. of Irvine, Calif. has developed a prosthetic valve fixed to a collapsible and expandable support frame that can be loaded into a delivery catheter. Such a bioprosthesis may be deployed in-situ minimally invasively through the vasculature at significantly less patient risk and trauma. A description of the CoreValve bioprosthesis and various embodiments appears in U.S. Pat. Nos. 7,018,406 and 7,329,278, and published Application Nos. 2004/0210304 and 2007/0043435. By using a minimally invasive replacement cardiac valve, patient recovery is greatly accelerated over surgical techniques. In the case of the CoreValve device, the support frame is made from shape memory material such as Nitinol. Other catheter-delivery valve replacement systems use stainless steel, or do not rely upon a rigid frame.
As demonstrated successfully to date, using a transcatheter procedure, percutaneous aortic valve replacement proceeds by delivering a prosthetic valve to the diseased valve site for deployment, either using a balloon to expand the valve support against the native lumen or exposing a self-expanding support in situ and allowing it to expand into place. With the latter, the self-expanding frame remains sheathed during delivery until the target site is reached. Advantageously, the frame may be secured to the catheter to avoid premature deployment as the sheath is withdrawn. In the CoreValve valve prosthesis, a hub is employed with two lateral buttons or ears around each of which a loop or alternatively a frame zig may reside during delivery. The internal radial force of the sheath keeps the frame compressed against the catheter, including the frame zigs in place around the lateral buttons. The catheter generally comprises at least two tubes, an inner tube that carries the prosthesis and a central tube that carries the sheath, permitting the sheath to move relative to the prosthesis.
As with traditional cardiovascular interventional therapies, transcatheter device deployment may proceed retrograde against normal blood flow, or antegrade, with blood flow. For percutaneous aortic valve replacement, entry through the femoral arteries proceeds in a retrograde format up through the iliac, the descending aorta, over the arch and to the native annulus. In some cases, entry has been made closer to the arch; for example through the left subclavian artery. Antegrade procedures have been performed where delivery takes place through the venous system transeptally to the native aortic annulus. More recently, transapical procedures have been performed whereby a cardiac surgeon delivers a catheter through the patient's chest wall, then through the exposed left ventricle apex and then to the target site.
With retrograde deployment, it is generally desired that the catheter be advanced within the vasculature so that the device is positioned where desired at the annulus site. With some embodiments under development, the desired site is the annulus itself With the CoreValve device, the desired site extends from the annulus to the ascending aorta, given its relative length. In the transfemoral approach, when the CoreValve device is positioned at the desired site, the sheath is withdrawn to the point where the inflow end of the device (preferably positioned at the native annulus) expands to engage and push radially outwardly the native valve leaflets. The sheath continues to be withdrawn proximally as the prosthesis continues to expand as it is exposed until the sheath covers just the outflow portion of the prosthesis still secured to the hub ears. Any readjustment of the axial position of the device in situ can be made during this process based upon electronic visual feedback during the procedure. Once well positioned, the sheath is fully withdrawn, the device fully expands in place, and the catheter is withdrawn through the center of the device and out through the vasculature. While it would be possible to deploy the prosthetic device such that the sheath could be withdrawn distally so that the outflow end of the prosthesis deploys first, such an arrangement would require advancing distally the central tube of the catheter connected to the sheath distally. In the case of transfemoral retrograde delivery, that would cause the central tube to project well into the left ventricle, which is not desirable. In a antegrade approach, for example transapical delivery, the reverse situation exists. There it is more desirable to advance the sheath distally to expose the inflow end of the prosthesis at the native annulus first. The native anatomy can accommodate this distal deployment because the central tube carrying the sheath is advanced up the ascending aorta towards the arch. Like the retrograde approach, once the valve prosthesis is fully deployed, the catheter may be withdrawn through the center of the prosthesis and removed through the apex of the heart.
Regardless of the direction of approach, with self-expanding frame technology, it is sometimes observed that even well-placed prosthetic valves inadvertently shift from the intended target site a small distance during the final delivery stage. The valve may still function effectively, but it is not optimized when, for example, the valve is placed so that it projects more than desired into the left ventricle. If the frame is implanted too low into the left ventricle, there is a risk of paravalvular leak where a portion of the blood ejected from the ventricle returns through the frame below the annulus.
In doing so, the catheter may sometimes inadvertently advance into the left ventricle for one of several possible reasons. One theory is that conical expansion of the zigs of the frame may influence positioning by following the path of least resistance until the inflow section is completely deployed in both the annulus and the ascending aorta may cause the prosthesis to shift. Another is the friction between the catheter sheath and the vessel wall, which may limit retraction of sheath even though the operator is pulling on it through the handle button. Consequently, the valve is pushed distally through the forward action of the plunger rather than the valve remaining stationary relative to the target site by the retraction of the sheath in the proximal direction. If the valve is not fully deployed (i.e., the sheath is not fully retracted) so that the valve frame is still secured to the catheter, axial adjustment is still possible. This is known as dynamic catheter positioning. In some cases, however, it is not determined until after full deployment that the frame is deployed amiss. In that circumstance, a repositioning procedure might need to be taken to correct placement. While possible in one of several different ways, it adds a level of complexity to the medical procedure that would be preferably avoided if possible. The problem is exacerbated because with transcatheter delivery, unlike surgical implantation, the clinician is unable to directly see the target site and must rely upon videographic technology to assess appropriate placement of the prosthesis.
One solution is to split the sheath into two discrete sections; a proximal section and a distal section. Doing so permits a controlled deployment that is central to the device, rather than at the distal or proximal end as with a single sheath. In U.S. Pat. No. 7,238,197, Seguin et al. have suggested such an arrangement, without any specificity or demonstration. See col. 14:36-42. No mention is made of the benefits or advantages of doing so, nor the particular configuration. Another example is shown in U.S. Pat. No. 7,022,133 to Yee et al. However, the distal and proximal portions are overlapping. Moreover, that disclosure does not address the issue regarding minimizing inaccurate deployment once the prosthesis is positioned at the target site. With an ill-configured split sheath arrangement, it has been observed that withdrawal of the catheter post-deployment may cause the proximal portion of the distal sheath to cause trauma to the native lumen where the vasculature is arcuate, such as the aortic arch. The problem may be encountered regardless of whether delivery proceeds antegrade or retrograde.
It is therefore desired to provide a transluminal catheter that enables prosthesis implantation accurately at the target location without the need for dynamic catheter positioning upon sheath retrieval. It is expected that, with such a solution, the prosthesis implantation procedure would become easier to manage with the desirable result that final positioning becomes more consistent.